The invention relates to a method for X-ray analysis of a sample in a particle-optical apparatus, in which:
a) two neighboring holes are formed in the sample, resulting in a separating wall between the holes,
b) a first side of the separating wall is irradiated by means of a beam of electrically charged particles, in response to which X-rays are generated in the separating wall, and
c) said X-rays are detected.
A method of this kind is known from a publication called xe2x80x9cFOCUSED ION BEAM SAMPLE PREPARATION FOR HIGH SPATIAL RESOLUTION X-RAY MICROANALYSISxe2x80x9d in Proc. 1995 5th. int. symp. on the physical and failure analysis of integrated circuits (5th IPFA 1995 Singapore), pp. 40-48.
In order to determine the elementary composition of a material, a sample of the relevant material can be irradiated by means of charged particles of adequate energy (for example, electrons), in response to which the sample emits X-rays whose wavelength distribution is characteristic of the chemical elements occurring in the relevant material. Such a method can be used not only for determining the elementary composition of the material, but also for determining the variation of the concentration of a given element as a function of the depth in the specimen, or for the detection of defects in, for example, integrated circuits.
In order to achieve a favorable spatial resolution for said applications, a small cross-section may be imparted to the irradiating beam, so that only a limited zone is struck by the beam so as to emit X-rays. In that case it may be assumed that the X-rays detected originate from the zone of the sample on which the beam is incident. For particle beams having a cross-section of less than approximately a few micrometers, however, the dimensions of the zone emitting the X-rays are no longer determined exclusively by the beam cross-section, but to a substantial degree also by the scattering within the sample of particles incident on the sample. The scattering zone has approximately the shape of a pear whose longitudinal axis extends in the direction of incidence of the particle beam. The diameter of the scattering zone is then determined inter alia by the energy of the particles and amounts to from approximately 100 nm (for electrons of an energy of 2 keV) to 7000 nm (for electrons having an energy of 30 keV) in the case of a silicon sample. In this respect it is assumed that the cross-section of the beam is small in comparison with the scatter zone.
For analysis of samples it is also desirable to know the elementary composition of a sample at a given distance below the surface, that is, at a given depth in the sample. To this end, it is known to provide a pit-like recess in the sample, one of the walls of the pit or hole subsequently being exposed to the particle beam. However, upon such irradiation the spatial resolution is again limited by the scattering in the material of the sample.
In order to achieve a higher spatial resolution nevertheless, the cited publication proposes the formation of two neighboring pits or holes in the sample by means of a focused ion beam. This results in a separating wall of small thickness (order of magnitude of 100 nm) between the holes. A first side of the separating wall is then irradiated by an electron beam, in response to which X-rays are generated in the separating wall. Because of the small thickness of the separating wall, the electrons from the incident beam will hardly be scattered in the sample material but will pass more or less rectilinearly through the thin separating wall. The dimension of the scattering zone will then be approximately equal to the cross-section of the electron beam in the target plane.
It is a drawback of the known method that the electrons having traversed the thin separating wall lose only little energy while doing so and hence reach, via the hole on the other (non-irradiated) side of the separating wall, the sample material and generate disturbing X-rays therein as yet.
It is an object of the present invention to provide a method of the kind set forth in which the described disturbing of the X-ray analysis is counteracted. To this end, the method in accordance with the invention is characterized in that, prior to the irradiation of the separating wall, the hole situated to the other side of the separating wall is filled at least partly with a stopping material of a composition which deviates from that of the separating wall.
Because the composition of the stopping material deviates from that of the material to be analyzed, the X-rays generated therein will have a different spectral composition in comparison with the material to be analyzed, so that the X-rays emanating from the stopping material can be distinguished from the desired measuring signal. This is the case notably when the X-ray response of the stopping material is known. In that case it often suffices even to neglect the spectral lines of the stopping material in the overall detected spectrum, or to subtract the known spectrum of the stopping material from the overall spectrum.
In conformity with one version of the method in accordance with the invention the separating wall contains silicon and the stopping material is formed essentially by a solid element having an atomic number higher than 71. This version offers the advantage that such solid elements are mainly heavy metals in which the electrons having passed the separating wall travel only a small distance, and hence give rise to a small scattering zone only. As a result, X-rays originating from this zone, in as far as they still have a disturbing effect, will affect the spatial resolution to a minor degree only. Preferably, platinum is chosen as the stopping material from the above group, because it exhibits a weak chemical and/or physical interaction with the sample material and can be readily deposited.
In conformity with a further version of the method in accordance with the invention, the separating wall contains silicon and the stopping material is formed essentially by a solid element having an atomic number lower than 13. This version offers the advantage that said elements exhibit a comparatively low X-ray fluorescence and background radiation of low intensity. Moreover, a significant part of the radiation caused by such light elements is absorbed in optical elements in the optical path of the X-rays, for example, by the window of the X-ray detector. Preferably, carbon is chosen as the stopping material from the above group, because it exhibits a weak chemical and/or physical interaction with the sample material and can be readily deposited, for example by decomposition of a jet of organic gas (containing carbon) in an ion beam.
In another version yet of the method in accordance with the invention the walls of the hole situated to the first side of the separating wall are at least partly lined with a lining material of a composition which deviates from that of the separating wall. In given circumstances it may occur that incident electrons are reflected on the separating wall and subsequently penetrate the sample material surrounding the hole at the entrance side of the separating wall. When the locations in the hole where such reflected electrons are to be expected are lined with said material, the X-rays generated by the reflected electrons can be distinguished from the desired X-ray signal. Considering the previously mentioned advantages of carbon, notably carbon can be chosen as the lining material. Moreover, carbon does not reflect electrons, or only an insignificant amount of electrons, so that the electrons intercepted by the carbon do not make any further contribution in disturbing the desired X-ray signal.